It is a sunny spring day; the water is sparkling, dotted with the white sails of jauntily leaning yachts and the green islands that speckle the US-Canada border. Welcome to the frontline of a vicious multibillion-dollar drug war.

A high-powered grey patrol boat with a three-man crew from the US department of homeland security buzzes across this Pacific idyll like a frenetic killjoy, boarding sailboats, disrupting jolly outings on family motor launches and even accosting tiny sea kayaks.

In theory, the crew's primary task is to stop terrorists infiltrating the US. Ever since Ahmed Ressam, an Algerian militant, was caught a few miles from here in December 1999 with more than 45kg (100lb) of explosives in the boot of his car, border patrols have been braced for the next episode. One of the crew wears a radiation detector at all times.

Since then, however, the homeland security patrol has been finding mainly marijuana on the boats they search - industrial quantities of a potent strain known as BC Bud, named in honour of the Canadian province where much of it is grown, British Columbia.

More than 900kg (1,984lb) of BC Bud is thought to reach the US market every year. The whole industry is thought to be worth $7bn (£3.7bn).

The product surges into the US like water flowing off a mountain, finding its way through every crack. It is dropped by small planes or helicopters into the raspberry fields and parks of Washington state. It is walked across the mountain forests in backpacks, stashed among frozen berries and driven in articulated lorries or in the back of vans on country roads. Or it comes by sea, on a flotilla of unassuming watercraft.

"See those boats. That's what BC Bud boats look like," said Kevin Anderson, one of the patrol's marine enforcement officers after boarding and searching a sailboat and a small motor cruiser, and finding nothing more menacing than an expired sailing licence.

The crew have been paying special attention to kayaks since last year, when a Canadian junior Olympic champion was caught putting his skills to lucrative use plying the sea border that runs through the Strait of Georgia. His boat was weighed down with his country's finest marijuana.

He was unlucky to get caught. On a fine summer's afternoon there can be 10,000 pleasure boats in the archipelago that forms the coastal borderline, and just one patrol.

BC Bud is so well thought of on the west coast it has been known to trade at the same price as cocaine, more than $3,000 a pound. In fact, it is commonly bartered for cocaine and guns, which travel in the opposite direction, north into Canada, making it a less safe and predictable place - and more like America - every day.

Drive-by killings are on the rise in the Vancouver area, as are house invasions, by which one gang seeks to take over another's marijuana crop without the bother of grow lights and hydroponic cultivation.

About a month ago four officers of the Royal Canadian Mounted Police were shot dead when they stumbled on a BC Bud-growing operation - the most Mounties lost in one day since the middle of the 19th century.

"It showed Canadians that the people who have grow-ops [growing operations] aren't all nice guys with mom-and-pop operations," said Inspector Paul Nadeau, the head of the force's coordinated marijuana enforcement team in British Columbia. "Nothing could be further from the truth. Every single criminal organisation in the region is involved."

It is a big pie everyone seems to want a slice of. A lot of the smugglers caught on the border are from ethnic Indian and Pakistani gangs in Canada. Many of the 50,000 grow-ops thought to be hidden across British Columbia are run by Vietnamese clans.

But police on both sides of the border say some of the biggest organisations coordinating the trade are chapters of the Canadian Hell's Angels.

Joseph Giuliano, the deputy chief patrol agent at the Blaine border post, has been watching them evolve from gangs to corporations. "They mostly farm out the dirty work," he said. "They have become administrators, bureaucrats, executives. The old days of them driving a Harley in a leather jacket are gone. Now they wear a three-piece suit and drive a Mercedes."

As the organisations behind BC Bud smuggling have grown larger, their operations have become more sophisticated, and the battle of wits at the border has become a technological race.

Mr Giuliano's patrols put sensors down along the border which send signals to a central command post in Blaine, generating a computerised voice alert announcing where there is movement and in what direction. Agents can then train one of 32 cameras on the area to determine whether a smuggler is making a crossing or a cow has gone astray.

The smugglers have equipped themselves with night-vision goggles and metal detectors in an attempt to locate the sensors under the cover of darkness. They also conduct surveillance operations watching the border patrols and testing their reaction times to the sensors, and intercepting radio messages with computerised scanners.

"They even have their own scientific sorts working on the capabilities of our gamma ray machines at the border," Mr Giuliano said. "They're testing what has similar density as BC Bud so that it's invisible." Apparently, frozen raspberries [this is one of the world's premier raspberry regions] come pretty close."

The cat and mouse game can also be as low-tech as a smuggler in a pick-up truck timing a run across the 45cm (18in) ditch that marks much of the land border in the north-west. Smugglers will swerve off the road over the ditch and gun their cars through rows of raspberry canes.

Until now, the casualties in this contest have been low but the deaths of the four Mounties have strained nerves. The stakes are getting higher and there are more guns involved.

Barbara Kremzner, a border patrol agent who drives around the Blaine area alone trying to stop cars getting across, said: "You never know what it's going to be. It gets hairy."

"We have a big problem on our hands," said Leigh Winchell, the special agent in charge of immigration and customs enforcement in Seattle. "Whenever that much money is involved, crime-related money, the violence follows.

"Huge quantities of cocaine and firearms and bulk cash are going north. The Canadians are dealing with a murder rate that is growing exponentially. I can't help but believe that if the violence continues to grow there, it will grow here."

Battle of BC Bud

The choice of British Columbia, Canada, for the cultivation of high-potency BC Bud is due in large part to its location. Sharing the world's longest undefended border with one of the world's largest drug markets - the US - Canada in general and British Columbia in particular is a logical haven for traffickers of cannabis.

Sophisticated cultivation techniques are used in British Columbia to maximise production. The plants are grown in large greenhouses where temperature, light and nutrients are carefully controlled, enabling growers to produce up to six crops a year.

Drugs cultivated in the western Canadian state are smuggled to Seattle and Portland, from where they can be distributed easily to the US's main marijuana markets, mainly using the highways which link large cities along the west coast from Canada to Mexico.

The US's office of national drug control policy estimates that between 75% and 85% of cannabis grown in British Columbia is exported to the US, where drug dealers can fetch higher prices than in Canada. The scale of illegal activity along the 4,000-mile border has prompted a big increase in US funding for stricter customs searches and more border patrols. Some traffickers have reacted by seeking safer markets in eastern Canada.

There is some debate over the relative potency of BC Bud, harvested from female plants. American enforcement agencies claim the drug contains between 15% and 25% of the active ingredient tetrahydrocannabinol, or THC - twice as strong as varieties from the US and up to six times stronger than cannabis from Colombia and Mexico. But campaigners accuse the authorities of stoking up panic about the drug and say the reality is closer to between 5% and 8%.